Many industrial and commercial operations create exhaust streams which contain toxic or odorous organic compounds. Examples of such operations include paint spraying, plastic molding, gluing, rendering, and chemicals manufacturing. Commercial facilities such as restaurants, printing shops, cleaners, and bakeries also produce gas streams which when emitted to the atmosphere might be undesirable under increasingly stringent environmental laws. As concern about these hydrocarbon emissions grows and regulatory restrictions become more prevalent, the need for devices and processes to control the levels of volatile organic materials released into the atmosphere becomes more acute.
When the concentrations of volatile organics in an exhaust stream are high or when the emission sources are a part of a large industrial complex, control technologies have been developed to remove these materials from the streams. Sophisticated and large conventional thermal catalytic or scrubbing systems are the norm for removing volatile organic materials from waste streams containing high concentrations of organics. However, when the concentration of organics in the stream is quite low, large volumes of gas must be handled to capture the relatively small amounts of volatile organics. Additionally the packed beds of adsorbent used in the conventional technology require large gas handling devices to push the gas through those beds. These facts contribute to the high costs of conventional systems and the fact that many of them are not cost effective. In any event, such devices are disclosed in the following:
U.S. Pat. No. 3,930,803 to Winter shows a process and an apparatus for purifying a gas flow having combustible and vaporous or gaseous impurities. The air or gas flows through an adsorption filter. The adsorption filter may contain a material such as activated charcoal, silica gel, or zeolite. When the impurity level in the adsorption filter reaches a predetermined level of impurities, the impurities are desorbed in counterflow by a hot inert gas generated through the stoichiometric combustion of hydrocarbons. The desorbate which is carried away by the inert gas is then subsequently burned with additional air or with oxygen.
U.S. Pat. No. 4,088,460 to Winter discloses a process and apparatus similar to that found in Winter '803. This document, however, shows an improvement in which a protective gas generator is used to produce an inert gas which is then introduced into the adsorption zone prior to the introduction of hot inert gas. This protective gas limits the potential for the presence of an explosive mixture in the adsorption zone as the hot gas is introduced.
U.S. Pat. No. 4,565,553 to Nowack shows an industrial painting method and system having a washer, dry-off oven, spray booth, and bake oven. Solvent paint vapors from the spray booth are concentrated in a filter which is subsequently degassed by a flow of hot air from the dry-off oven. The degassed vapors are then admixed with the atmosphere of the bake oven. The bake oven atmosphere is continuously circulated to the washer and passed through the burner of the washer to incinerate solvent vapors. The gases produced by the incineration are carried to the dry-off oven to form the atmosphere there. The principal exhaust from the overall system is from the dry-off oven and is substantially free of solvent vapors and is of a fairly low temperature. The filters used to adsorb the solvent contaminants from the air may be carbon, silica gel, activated alumina, molecular sieves, and certain clays. Activated carbon is preferred. The solvent vapors are released from the filter by a countercurrent flow of heated air.
U.S. Pat. No. 4,737,164 discloses a process for recovering various volatile organic and inorganic impurities from gases using an adsorber which contains two layers of a fibrous activated carbon configured in the form of a cylinder. The cylindrical carbon layers are separated from each other by in insulating wall. The residual solvent in the air is adsorbed as it passes through the fibrous activated carbon. The solvent is desorbed by use of a low pressure steam flow or other inert gas while concurrently applying an electric current to the carbon fiber adsorption bed.
U.S. Pat. No. 4,846,852 to Schweitzer et al. discloses a method and apparatus for separating and recovering volatile solvents (such as that found in the exhaust of dry-cleaning machinery, metal degreasers, installations for cleaning electronic parts, solvent baths, etc.) by passing the exhaust gas alternatively through one of two beds of molecular sieve packings capable of adsorbing water vapors contained therein and through one of two beds of molecular sieve packings composed of a material suitable for adsorbing the solvent vapors contained therein. Heated air or inert gas is passed through the beds not in use to desorb the water vapor and solvent vapors and thereby regenerate the molecular sieve packings. The water and solvent are then condensed for ultimate recovery as liquids.
U.S. Pat. No. 4,863,494 to Hayes discloses a process for removing volatile organic compounds from air by passing the air through a bed of a divinylbenzene beads. The beads adsorb such materials as gasoline, benzene, chlorinated solvents, various alcohols, esters, acid gases, and the like. The beads are regenerated by passing a heated gas through the bed at a temperature as high as 290.degree. C.
U.S. Pat. No. 4,902,311 to Dingfors et al. shows a method and an apparatus for removing various organic substances, primarily solvents, from industrial ventilation air contaminated with such substances. In the process, air containing the solvent is passed through an adsorbent comprising macroporous polymeric particles in a fluidized bed. The particles are fed continuously to the bed and then are fed continuosly to a desorption column. In the desorption column the solvent is desorbed from the adsorbent by the use of heated air. The desorbed material is drawn off to a cooler.
None of these patents suggest a process or a device in which an electrically conducting adsorbent or catalyst support is solely utilized to remove hydrocarbon contaminants from a highly dilute gaseous source.
Another choice for volatile organics removal is a continuous combustion unit to destroy the organics by oxidation or combustion at high temperature. At the level of volatile organics found in many waste streams, the heat from the combustion reaction is simply not sufficient to sustain the high temperatures necessary to destroy the organic material. Thus, unless the gas stream is already quite hot, a combustion unit requires additional energy input (most often by burning a fuel) for operation. This additional fuel requirement makes operating costs high and renders this option unattractive for small operations or dilute organic sources.
Although the use of activated carbon beds or cartridges is known in certain industries as a suitable approach for adsorbing small amounts of volatile organics as the stream passes through the beds, methods for regenerating the bed are difficult and often create an environmental hazard when releasing the organic from the bed. The cartridges are simply often discarded. Where steam stripping or thermal cycling is used, the energy costs are high. Typical of such devices are:
U.S. Pat. No. 3,274,755 to Montagnon et al. discloses a process in which a volatile solvent used in a dry-cleaning or de-greasing operation is absorbed from a vent stream using a carbon or charcoal bed. The solvent is said to be a chlorinated hydrocarbon. The carbon bed is regenerated by use of steam sparging in the direction opposite to the flow of the solvent-laden air.
Kokai 52-65,175 issued to Mitsui Kikinzoku Kogyo K.K. describes a process and an apparatus for treating a waste gas stream to remove low concentrations of combustible gases. The waste gas is passed through an adsorption zone which, in the example, is said to be a bed of activated carbon. In that example, toluene was removed from a gas stream. Some amount of combustible gas is apparently expected to pass through the activated carbon bed and into the following oxidation reactor. The oxidation reactor is then able to handle the lower concentration of the combustible component. Apparently this process has the effect of smoothing the concentration of the combustible component to a nearly constant level throughout the flow of the gas stream from the upstream batch reaction contemplated in the Kokai.
Again, neither of these documents utilizes a source of heat in which the adsorbent and the catalyst are found on an electrically conductive, heat-producing support.
Disclosures which do utilize a voltage differential in an adsorption process include the following.
U.S. Pat. No. 4,094,652 to Lowther discloses a system and process for regenerating an adsorbent bed of a dielectric absorbent particles by applying to those particles a high voltage electrical field (e.g., 0.2 to 500 Kv/cm) to separate substantially all of the adsorbed moisture as molecular water. The voltage may be applied either as a direct current or a low frequency pulsing or AC current (0-1000 Hertz). The advantages are said to be that the amount of time and energy required in regenerating the bed are less than for conventional regeneration procedures.
Kokai 54-160,589 issued to Toho Beslon K.K. suggests an adsorption/desorption unit made up of fibrous activated carbon. The active carbon is in the form of fiber mat containing discrete metal fiber heaters within the mat. The gas containing the offending hydrocarbon is passed through the carbon mat and is adsorbed onto the individual fibers. When the capacity of the mat is reached, an electric current is passed through the discrete metal wires included in the mat so as to raise the temperature of the carbon to 100.degree. C. or 150.degree. C. The heating causes the adsorbed material to desorb. In the example, an air flow containing trichloroethylene was treated using the process. The process had an overall recovery efficiency of 85%.
In summary, none of the noted disclosures show a process or device in which an adsorber and a catalytic oxidation reactor utilize an electrically conductive adsorbent bed or oxidation bed variously to raise the temperature of the adsorbent to desorb the hydrocarbon or to raise the temperature of the catalyst to support the oxidation reaction.
Our invention permits the use of low cost equipment. The energy costs are low. For many variations of the invention, the pressure drop across the bed or beds is low and the required gas-moving or gas-handling equipment is, by comparison, not expensive. The low pressure drop allows installation of the bed in existing exhaust blown vent systems.